The Layer 2 Radio Link Control, RLC, protocol for the Evolved Universal Terrestrial Radio Access, E-UTRA, or Long Term Evolution, LTE, radio standard is described in specification 3GPP TS 36.322 version 13.0.0. Methods for optimizing the RLC protocol in terms of overhead and enabling high bit rates (up to 25 Gbps) are currently being standardized in 3GPP.
The current RLC Sequence Number (SN) length defined for LTE of 10 bits is sufficient to support the peak data rates on RLC layer for legacy applications over early LTE implementations. However, LTE continuously evolves and has now extended its support to much higher peak rates using Carrier Aggregation of up to 32 LTE carriers with the resulting maximum peak rate support of 25 Gbps for which a 16 bits' SN length configurable may be used.
The throughput capacity of RLC is limited by the so called window stall phenomenon. This is the result of that the transmitting RLC entities are not allowed to send new RLC Protocol Data. Units (PDUs) and advance the RLC transmission window unless they have received RLC status reports at a sufficient rate. The maximum rate at which PDUs can be transferred is when the upper end of the RLC transmission window is reached at same time as the RLC status report for the lower end PDU is reached. The transfer of new PDUs will be suspended while such report is missing, and resumed when it arrives. The resulting intermittent transmission of application data due to the transmitting RLC entity periodically reaching the highest allowable SN in the window and temporarily ceasing transmissions clue to this limitation is termed an RLC window stall. The window size of LTE is defined as half the SN range, i.e. 512 in case of 10 bits' SN. The problem of RLC window stall is typically associated with Data Radio Bearers (DRBs) since data rates over Signalling Radio Bearers (SRBs) are practically much lower. However, in light of current 3GPP work for Data over Non-Access Stratum, DoNAS, a future may involve more frequent Service Data Units, SDUs, over SRB, and it might come to times when the present SN length used by RLC for SRBs may also come short with regards to the SDU rates associated to such transfer.